In the recent art of the silver halide photographic light-sensitive material (hereinafter referred simply to as light-sensitive material), techniques for a higher image quality and a lower silver consumption are progressing very fast. And for such purposes, there have been made various studies including monification of silver halide grains, enlargement of the aspect ratio of silver halide grains and use of monodispersed twin grains, from the viewpoint of the silver halide emulsion and manufacturing method thereof.
Techniques on silver halide grains having large aspect ratios are disclosed in Japanese Pat. O.P.I. Pub. Nos. 113926/1983, 113927/1983, 113928/1983 and 163046/1987. Silver halide grains disclosed therein are tabular grains having an aspect ratio larger than 8.
The term "aspect ratio" used here, which is applicable to a twin grain having two or more parallel twin faces, is given as a ratio of the diameter of converted circle to the space measured on a photograph thereof taken from a direction vertical to the twin face and the thickness between the two parallel surfaces of a grain.
One of the advantages of using silver halide grains having a large aspect ratio is that such grains have a per volume surface area larger than those of regular crystalline silver halide grains such as octahedrons, tetradecahedrons and cubes; accordingly, they can adsorb much sensitizing dyes on the surface, providing a higher sensitivity.
Minifying the grain size of silver halide cannot be dispensed with for a higher image quality and a lower silver consumption in light-sensitive materials. The grain size means the diameter of a circle converted in the same area from a projected image of a grain. The foregoing techniques can provide silver halide grains of high aspect ratios, but cannot make small and monodispersed silver halide grains.
Among patent applications published up to the present, ones disclosing a monodispersed twin grain emulsion include Japanese Pat O.P.I. Pub. Nos. 6643/1986 and 14636/1986. The technique described therein provides a monodispersed twin grain emulsion by carrying out Ostwald ripening after formation of nuclei to obtain an emulsion comprising fine monodispersed spherical seed grains and then growing the fine grains, hereinafter an emulsion comprising fine silver halide seed grains to be grown to suitable size grains is referred to a seed emulsion. The monodispersed twin grain emulsion prepared by this method has an advantage of being easily subjected to an optimum chemical sensitization, as compared with a multidispersed emulsion in which large grains and small grains are mixed with one another.
However, when such a seed emulsion is used, a highly monodispersed emulsion can be obtained, but an emulsion comprising small-size silver halide grains having high aspect ratio cannot be prepared.
In addition, a similar technique for making a monodispersed twin grain emulsion, which has an Ostwald ripening process after nucleus formation, is disclosed in Japanese Pat. O.P.I. Pub. Nos. 158426/1989, 213637/1989 and 838/1990. In this technique, a low molecular weight gelatin is used at the time of nucleus formation. However, the low molecular weight gelatin is insufficient in properties as a protective colloid and liable to aggregate silver halide grains, preventing stable manufacture of light-sensitive materials Further, it is more expensive than the relation ordinarily used in preparation of silver halide emulsions.
Japanese Pat O.P.I. Pub No. 28638/1990 discloses, in its Example 1, an emulsion containing small and high-aspect-ratio twin silver halide grains having an average grain size of 0.52 .mu.m and an average aspect ratio of 9.5, which was obtained by forming nuclei at 30.degree. C. and ripening them while raising the temperature to 65.degree. C. But the emulsion's variation coefficient of grain size distribution is as high as 30%.
The present inventors made a follow-up study by carrying out Ostwald ripening at a temperature higher than that in the grain formation as disclosed therein. But, a monodispersed spheric twin seed emulsion as described in Japanese Pat. O.P.I. Pub. No. 6643/1986 could not be obtained, and grains in the seed emulsion were too large to give a desired monodispersed twin grain emulsion.